net/rpc: fix data race in benchmark Fixes issue 2781.
net/rpc: fix data race in benchmark
Fixes issue 2781.

R=golang-dev, rsc
CC=golang-dev, mpimenov
http://codereview.appspot.com/5577053
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go